1. Field of the Invention
The present invention relates to a printer, and more particularly to a thermal transfer type color printer with a line thermal head.
2. Prior Art
Various types of printers are known which melt or sublimate color inks on a surface of a transfer sheet by a thermal head to successively apply the color inks to a recording sheet to print a color image thereon. FIG. 30 shows one of such conventional printers which comprises a clamper 403 for holding a front end portion of a recording sheet 401, a platen roller 402 for winding the recording sheet 401 held by the clamper 403, a pair of rollers 405, 406 for winding front and rear sides of the recording sheet 401, and a thermal head 408 supported by a shaft 407 to be pressed against the platen roller 402 with a transfer sheet 404 and the recording sheet 401 being interposed therebetween at the time of the printing mode. The transfer sheet 404 has color inks such as yellow, magenta, cyan and black which are successively applied thereon to be stripped in the longitudinal directions. The width of each of the ink-applied stripes substantially corresponds to that of one picture plane. Further, the thermal head 408 has a heating section comprising a plurality of heating elements successively arranged in a line in parallel with the shaft of the platen roller 402. In response to supply of a predetermined current signal, each of the plurality of heating elements causes heat simultaneously to effect a printing of an amount corresponding to one line.
In a printing operation, the recording sheet 401 fed from a sheet-feeding tray (not shown) is held on the platen roller 402 by the clamper 403, and then the platen roller 402 is rotated by a motor (not shown) while a current signal is supplied to the thermal head 408 pressed against the platen roller 402 in a state that the transfer sheet 404 is wound around the wind roller 405 by a wind motor (not shown), thereby transferring the color ink of the transfer sheet 404 onto the recording sheet 401. After completion of the first-color printing operation, the platen roller 402 is rotated at a higher speed to feed the recording sheet 401 to the recording start position, while transfer sheet 404 is wound around the wind roller 405 so that a portion having the next color ink comes into contact with the recording sheet 401 to perform the similar printing operation again. With such a printing operation being repeatedly executed with respect to the same recording sheet 401, the respective color inks on the transfer sheet 404 are overlapped on the surface of the recording sheet 401 to form a color image thereon. After completion of the printing operations by all the color inks, the thermal head 408 is raised up about the shaft 407 clockwise in the illustration to be separated from the platen roller 402 and the recording sheet 401 printed is discharged into a sheet-discharging tray (not shown), thus completing the printing operation.
However, this FIG. 30 arrangement has a disadvantage in that the diameter of the platen roller 402 becomes large with increasing size of the thermal head 408, thus increasing the cost of the printer.
FIG. 31 shows another conventional printer. In FIG. 31, a recording sheet 401 is pressed against a platen roller 402 by pairs of sheet-feeding rollers 409 and 410 without holding the recording sheet 401 with respect to the platen roller 402. For printing, the recording sheet 401 is reciprocated in back-and-forth direction by the sheet feeding rollers 409 and 410.
Although the diameter of the platen roller 402 is small, this FIG. 31 printer will encounter difficulty in accurately setting the recording sheet 401 at the recording start position in each reciprocative movement of the recording sheet 401, thus deteriorating image quality derived from positional deviation of color images.
Furthermore, FIG. 32 illustrates a conventional printer which mainly includes a clamping unit 420 for clamping one end portion of a recording sheet 401, a carrying mechanism 421 for movably supporting the clamping unit 420 in the right and left directions (generally, in the secondary scanning directions of the recording sheet) in the illustration, and a recording unit 422. The clamping unit 420 comprises a clamper 423, a substantially L-shaped pulley block 424 for conveying the clamper 423, and a solenoid 426 fixedly secured to the pulley block 424. Here, the clamper 423 is rotatably fitted to a shaft 425. References 424a and 424b are pulleys attached to the pulley block 424. The tip portion of the recording unit 422 side of the clamper 423 is bent so as to appropriately clamp the recording sheet 401 and the other end portion thereof has the solenoid 426 which rotates the clamper 423. Furthermore, the carrying mechanism 421 comprises drive pulleys 436 and 437, a belt 428 stretched between the drive pulleys 436 and 437, and others. The belt 428 is disposed at the lower portion of the clamping unit 420 so as to be driven through belts b and c by means of a motor 427 fixed to the recording unit 422. The pulley block 424 is fixedly secured to the belt 428. Furthermore, below the belt 428 there is disposed a stage 429 for allowing the pulleys 424a and 424b fixed to the pulley block 424 to move in the secondary scanning directions of the recording sheet 401. The pulleys 424a and 424b are moved by means of an adequate mechanism (not shown). On the stage 429 there is provided a reflection type positioning sensor 430 which serves as detecting means for determining the original point (i.e recording start point) of the pulley block 424. When the motor disposed in the recording unit 422 is driven, the motor's power is transmitted to the platen roller 402 and the sheet-feeding rollers 409 and 410. These sheet-feeding rollers 409 and 410 guide the recording sheet 401 fed. Here, the feeding roller 409 is urged toward the feeding roller 410 by means of an appropriate mechanism (not shown) to slide on the recording sheet 401. Furthermore, on the feeding roller 410 attached to a fixed shaft, there is provided a one-way clutch whereby the recording sheet 401 is movable only in one direction. In the motor 427 there is provided an encoder, and rotation of the motor 427 is controlled with motor driver receiving a predetermined number of pulses from the encoder. The feeding roller 410 is arranged to be rotatable at a lower speed than the drive pulley 437 by changing the gear ratio so as to prevent the recording sheet 401 from being loosen on the travelling path. The platen roller 402 is disposed between the carrying mechanism 421 and feeding rollers 409, 410 to receive pressure applied to the thermal head 408. The thermal head 408 is arranged to be rotatable about a shaft 407 and is pressed against the platen roller 402 by rotation of an eccentric cam 431 so that pressure is evenly applied to the heating portion of the thermal head 408. Above the thermal head 408 there are provided a pair of transfer sheet rollers 405 and 406 for winding a transfer sheet 404. An intermediate portion of the transfer sheet 404 wound around the transfer sheet rollers 405 and 406 is disposed between the thermal head 408 and the platen roller 402. Reference numerals 432 and 433 are guide rollers provided at the front and the rear sides of the platen roller 402 for causing the transfer sheet 404 to smoothly travel.
According to the FIG. 32 conventional printer, in the reciprocative movement of the recording sheet 401, the recording sheet 401 is so accurate in positioning the recording start point to provide high-quality image and the diameter of the platen roller 402 can be reduced to allow using a small-sized thermal head 408 for cost reduction of the printer. However, this arrangement requires the carrying mechanism 421 for allowing the accurate positioning of the recording sheet 401. The provision of the carrying mechanism 421 increases the size and cost of the printer.